1. Field of the Invention
The present invention relates to excitation systems for synchronous dynamoelectric machines, and more particularly to a method and means for starting a synchronous dynamoelectric machine which is mechanically coupled to a large inertia load.
2. Description of the Prior Art
Certain prime movers, for example gas turbines which are designed to drive large generators, are not selfstarting and must be accelerated to nearly 60% of full speed whereupon the combustor can be ignited and the developed turbine power is sufficient to continue acceleration to full speed. Typically, a gas turbine is started by a clutch-connected device, usually an induction motor, starting turbine, or diesel engine. Each of these are geared through a speed increaser and a torque converter. When an induction motor starter is used, the required size of this motor is about 6% of the turbine generator rating. For example, for a turbine generator rating in the 70 to 80 megawatt range the power rating of the starting motor is approximately 1500 horsepower. Current in-rush of such a motor is usually about six times the rated motor KVA, and such large KVA demand is not always available from the system that the oncoming generator is being connected to.
Also, in a plant including several gas turbine generators, each turbine generator requires its own starting motor which remains idle except for use during a brief starting period. In addition suitable control equipment must be provided for the motors, and torque converters with the necessary cooling system, speed increasing gears, and clutches are also required for the complete starting system. Because of the expense associated with providing and maintaining a separate starting motor for each turbine generator, there is substantial interest in providing an alternative starting arrangement which can serve multiple generator prime mover units.
It has been proposed to start prime movers which are not self-starting by using the generator itself as a starting motor, as in the patents to Erikson, U.S. Pat. No. 3,132,297; and Schonebeck et al., U.S. Pat. No. 3,591,844. This has been done with relative success with small machines, such as aircraft generators driven by internal combustion engines, but the method and structure as suggested in those references is neither practical nor desirable in the case of large synchronous generators such as are used in plants supplying power to utility systems. A suitable starting arrangement in which the exciter for a synchronous generator is used to start a large prime mover such as a gas turbine is disclosed in Kilgore et al. U.S. Pat. No. 3,809,914. However, this arrangement relies upon a combination of switches and slip rings to cause the exciter alternator to operate as a variable speed, wound rotor induction motor for starting purposes. Such an arrangement cannot be effectively utilized for starting a synchronous dynamoelectric machine of the type having a brushless exciter since no slip rings or other sliding electrical contacts are employed.